Celestia/Glossary

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This is a glossary of Celestia terminology. Many of the terms used with Celestia are very similar to the corresponding astronomical terminology. For a glossary of astronomical terms, see the Astronomical Terms: Appendix to the Wiktionary

A text file used to define objects and their positions. Different catalogs are used to define a Star (STC), a planetary system which orbits around a Star (SSC) and objects not associated with a Star (DSC).

Julian dates are a continuous count of days and fractions since noon Universal Time on January 1, 4713 BCE (on the Julian calendar). Almost 2.5 million days have transpired since this date. Julian dates are used in Celestia's SSC and STC catalogs for the fields Epoch, Beginning and Ending. These fields use 64-bit floating point (double precision) variables, and can represent a Julian date to about 1 millisecond of precision. The time scale that is the basis for Julian dates is Universal Time. Starting with v1.5, Celestia uses TDB internally and UT for its on-screen display. 0h UT corresponds to a Julian date fraction of 0.5: Julian days go from Noon to Noon instead of from Midnight to Midnight. This is so that the date doesn't change in the middle of nocturnal observations.

An image file providing information about the visual characteristics of an object.

A mathematical transform between the geometry of a spheroidal object and a flat surface. The mapping known as "simple cylindrical projection" or "Plate Carré" is used by Celestia to map between flat images (surface texture files) and spherical planets and moons.

A (usually moving) invisible position in space defined in an SSC catalog file, around which other SSC objects can orbit. A ReferencePoint has only positional characteristics. It cannot be used to provide any orientation information.

Spacecraft, Planet, Instrument, C-matrix and Events kernels; not to be confused with Simulation Program with Integrated Circuit Emphasis. A type of NASA trajectory information file. Use of SPICE ketnels was introduced in Celestia v1.5.0.